System and method of selecting antennas based on signal-to-noise ratios and signal quality values

ABSTRACT

A system includes a receiver, a processor and a decision device. The receiver receives a first signal via a first antenna, and one of the first signal and a second signal via a second antenna. The processor determines: based on the first signal as received at the first antenna, a first signal-to-noise ratio (SNR) and a first signal quality value; and based on the one of the first and second signals as received at the second antenna, a second SNR and a second signal quality value. The decision device selects one of the first and second antennas based on (i) a difference between the first SNR and the second SNR if the first SNR is greater than the second SNR, or (ii) the first signal quality value and the second signal quality value if the difference between the first SNR and the second SNR is less than a predetermined threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/567,832 (now U.S. Pat. No. 8,243,778) filed Sep. 28, 2009, which is acontinuation of U.S. patent application Ser. No. 12/231,341 (now U.S.Pat. No. 7,599,424) filed Sep. 2, 2008, which is a continuation of U.S.patent application Ser. No. 11/449,956 (now U.S. Pat. No. 7,421,012)filed Jun. 9, 2006, which is a continuation of U.S. patent applicationSer. No. 09/991,473 (now U.S. Pat. No. 7,099,380) filed Nov. 16, 2001.The disclosures of the above applications are incorporated herein byreference.

FIELD

The present invention relates to a technique for using antenna diversityfor wireless communication in a multipath signal environment, and moreparticularly to a technique for assessing channel quality based onmultipath-related factors.

BACKGROUND

Signal environments that include noise and/or interference present aproblem for communication systems that has been addressed using manytechniques, including signal amplification, signal encoding, filteringby frequency range, and spread-spectrum modulation. Another techniquethat has been often used is that of antenna diversity. Referring to FIG.1, in a communication system 100 that uses antenna diversity, at leasttwo separate antennas 105, 110 are used as inputs to a single receiversystem 115. For each data packet, the receiver 115 is designed to selectwhich antenna input is providing the best signal reception for each datapacket. Antenna diversity is most useful when operating in a wirelesscommunication system, such as a system that uses radio communicationtechniques.

Antenna diversity techniques for signal reception in noisy environmentsand interference environments are very well known in the literature andare the subject of many patents. For example, see U.S. Pat. Nos.5,077,753; 5,838,742; 6,061,574; 6,115,406; 6,215,812; 6,215,814;6,229,842; 6,240,149; 6,256,340; 6,259,721; and 6,278,726, and EuropeanPatent Office Publication Number 0 642 243 A1, the contents of each ofwhich are incorporated herein by reference.

Referring again to FIG. 1, one type of interference that is oftenencountered in communication systems is known as multipath interference.Multipath interference occurs when a signal propagates by more than onepath between the transmitter 120 and the receiver 115. For example, asignal may propagate directly by line-of-sight along paths 125 betweenthe transmitter 120 and the receiver 115, and it may also “bounce” offof the ground or some other reflective object en route along path 130between the transmitter 120 and the receiver 115. When a signal arrivesat a receiver after traveling by two or more paths, interference occursbecause of the different path lengths, which causes the transmissiontime to differ and thus leads to the two or more separate reception ofthe signal to interfere with each other. Furthermore, even if thereceiver is designed to filter out other signals based on frequency orcoding or modulation, the receiver will typically be unable todistinguish between the multiple receptions of the same signal, becausethe received signal is exactly the signal for which the receiver istuned and ready to receive.

Referring to FIGS. 2 and 3, in many communication systems that useantenna diversity to counteract the effects of noise and interference,signal-to-noise ratio (SNR) is the quantity that is typically measuredand used as the determinant in the selection of which antenna should beemployed. For example, SNR may be determined by measuring the amount ofgain applied by a variable-gain amplifier (VGA) to keep the circuitinput constant. A gain level 205 applied to noise may be compared with again level 210 applied after the start of a data packet, and the SNR ismeasured by taking the ratio between the two gain levels 205, 210. Asanother example, a signal power level 215 may be measured and comparedwith a noise power level 220, and the signal-to-noise ratio may becomputed by dividing the signal power level 215 by the noise power level220 (or, if the power levels are expressed in decibels, by subtractingthe noise level 220 from the signal level 215).

FIG. 3 shows a block diagram for a conventional communication system 300designed to select an antenna from two or more antennas in an antennadiversity scheme by using the signal-to-noise ratio as the determinant.The system 300 includes at least two receive antennas 305 that feed thereceived analog signal into a multiplexer 310. The signal passes througha low noise amplifier 315 before its frequency is converted to anintermediate frequency IF by a converter 320. The signal is thenprocessed by a variable-gain amplifier 325, and then down converted tothe baseband frequency by converter 330. The analog signal is digitizedby an A/D converter 335, and then filtered by a filter 340 to removesignals outside its frequency range. Finally, the digital signal passesthrough an automatic gain controller 345, which effectively sets thesignal level to a predetermined value and thereby measures the SNR. Thepredetermined value is set by using a control signal with a knownamplitude as a reference, so that the signal power at the input of theautomatic gain controller (AGC) 345 remains constant. The control signalis generated by the AGC 345, and then fed back to the variable-gainamplifier (VGA) 325 to adjust the gain level of the VGA 325. Based onthe measured SNR, an antenna selection is made by unit 350.

However, in a multipath signal environment, the system performance maydepend more upon characteristics related to the multipath nature of theenvironment than upon the SNR, especially because SNR is generally not areliable indicator of multipath effects. Hence, to optimize systemperformance, multipath factors are taken into account. Accordingly,there is a need for a new method of selecting a receive antenna in anantenna diversity system when operating in a multipath signalenvironment.

SUMMARY

In one aspect, a receiver is provided and includes antennas forreceiving a signal. The signal includes a data packet. The receiverincludes a correlator, a processor, and a decision device. Thecorrelator is configured to correlate a spreading code with a preambleof the data packet. The processor is configured to determine a signalquality value of the signal at each of the antennas. The signal qualityvalue at each of the antennas (i) is determined in response to thecorrelation of the spreading code with the preamble of the data packet,and (ii) includes a peak-to-average ratio value. The decision device isconfigured to select a first antenna of the antennas for reception ofthe signal based on the signal quality value determined at each of theantennas.

The disclosed implementations include selecting an antenna in an antennadiversity system for wireless communications in multipath signalenvironments.

In one aspect, a communication system is provided for communicating datapackets. The system includes a receiver subsystem. The receiversubsystem includes a first antenna, a second antenna, a spread-spectrummodulator, and a signal quality measurement device. The device isconfigured to measure signal quality values corresponding to each of thefirst and second antennas for each data packet and to select an antennaon the basis of the measurements.

The communication system may also include an analog-to-digitalconverter, a filter, and an automatic gain control unit. Theanalog-to-digital converter may be configured to extract a predeterminednumber of sample values from each incoming data packet. The signalquality measurement device may be configured to measure signal qualityvalues other than SNR by computing peak-to-average ratio values bydividing a peak sample value by an average sample value. The averagesample value is determined by averaging the predetermined number ofsample values for each incoming spreading codeword. The peak samplevalue is determined by selecting the maximum of the predetermined numberof sample values for each incoming spreading codeword. Thespread-spectrum modulator may be configured to employ direct sequencespread spectrum modulation, such as an eleven-chip Barker codemodulation. The communication system may conform to either an IEEE802.11 standard specification or an IEEE 802.11(b) standardspecification.

In another aspect, an antenna array is disclosed and provides antennadiversity to a communication system. The array includes at least twoantennas. The system includes a spread-spectrum modulator and a signalquality measurement device, and the system is configured to receive datapackets. The signal quality measurement device is configured to measuresignal quality values corresponding to each of the at least two antennasfor each data packet and to select an antenna on the basis of themeasurements.

The communication system may also include an analog-to-digitalconverter, a filter, and an automatic gain control unit. Theanalog-to-digital converter may be configured to extract a predeterminednumber of sample values from each incoming data packet. The signalquality measurement device may be configured to measure signal qualityvalues by computing peak-to-average ratio values by dividing a peaksample value by an average sample value. The average sample value isdetermined by averaging the predetermined number of sample values foreach incoming spreading codeword. The peak sample value is determined byselecting the maximum of the predetermined number of sample values foreach incoming spreading codeword. The spread-spectrum modulator may beconfigured to employ direct sequence spread spectrum modulation, such asan eleven-chip Barker code. The communication system may also conform toeither an IEEE 802.11 standard specification or an IEEE 802.11(b)standard specification.

In yet another aspect, a method and an apparatus for selecting anantenna from an antenna diversity array in a communication system areprovided. The antenna diversity array includes at least two antennas.The method includes the steps of modulating an incoming signal with aspread-spectrum type modulation, measuring a signal quality value foreach antenna in the antenna diversity array, and selecting an antennafrom the antenna diversity array on the basis of the measured signalquality values.

The communication system may include an analog-to-digital converter, afilter, and an automatic gain control device. The analog-to-digitalconverter may be configured to extract a predetermined number of samplevalues from each of incoming signal packets. The step of measuring asignal quality value for each antenna may include computingpeak-to-average ratio values by dividing a peak sample value by anaverage sample value. The average sample value is determined byaveraging the predetermined number of sample values for each of theincoming spreading codewords. The peak sample value is determined byselecting the maximum of the predetermined number of sample values foreach of the incoming spreading codewords. The step of modulating mayinclude employing direct sequence spread spectrum modulation, such as aneleven-chip Barker code. The communication system may conform to eitheran IEEE 802.11 standard specification or an IEEE 802.11(b) standardspecification.

In still another aspect, a method and an apparatus for improvingreliability in a communication system when operating in a multipathsignal environment are provided. The communication system includes anantenna diversity array having at least two antennas. The methodincludes the steps of modulating an incoming signal with aspread-spectrum type modulation, measuring a signal quality value foreach antenna in the antenna diversity array, and selecting an antennafrom the antenna diversity array on the basis of the measured signalquality values.

The communication system may include an analog-to-digital converter, afilter, and an automatic gain control device. The analog-to-digitalconverter may be configured to extract a predetermined number of samplevalues from each of incoming signal packets. The step of measuring asignal quality value for each antenna may include computingpeak-to-average ratio values by dividing a peak sample value by anaverage sample value. The average sample value is determined byaveraging the predetermined number of sample values for each of theincoming spreading codewords. The peak sample value is determined byselecting the maximum of the predetermined number of sample values foreach of the incoming spreading codewords. The step of modulating mayinclude employing direct sequence spread spectrum modulation, such as aneleven-chip Barker code. The communication system may conform to eitheran IEEE 802.11 standard specification or an IEEE 802.11(b) standardspecification.

In yet another aspect, a method and an apparatus for mitigatingintersymbol interference in a communication system operating in amultipath environment are provided. The communication system includes anantenna diversity array having at least two antennas. The antennadiversity array includes at least two antennas. The method includes thesteps of modulating an incoming signal with a spread-spectrum typemodulation, measuring a signal quality value for each antenna in theantenna diversity array, and selecting an antenna from the antennadiversity array on the basis of the measured signal quality values.

The communication system may include an analog-to-digital converter, afilter, and an automatic gain control device. The analog-to-digitalconverter may be configured to extract a predetermined number of samplevalues from each of incoming signal packets. The step of measuring asignal quality value for each antenna may include computingpeak-to-average ratio values by dividing a peak sample value by anaverage sample value. The average sample value is determined byaveraging the predetermined number of sample values for each of theincoming spreading codewords. The peak sample value is determined byselecting the maximum of the predetermined number of sample values foreach of the incoming spreading codewords. The step of modulating mayinclude employing direct sequence spread spectrum modulation, such as aneleven-chip Barker code. The communication system may conform to eitheran IEEE 802.11 standard specification or an IEEE 802.11(b) standardspecification.

In still another aspect, a storage medium is provided for storingsoftware for implementing each of the methods described above. Thesoftware is computer-readable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a communication system that uses antenna diversity tomitigate noise and interference.

FIG. 2 illustrates a measurement of signal-to-noise ratio for a signalpropagating in a noisy environment.

FIG. 3 shows a block diagram of a conventional communication system thatuses signal-to-noise ratio to select an antenna for signal reception.

FIG. 4 shows an idealized digital signal after despreading in anenvironment having no noise and no interference.

FIG. 5 shows a sampled digital symbol in a multipath signal environmentafter despreading.

FIG. 6 illustrates a signal propagating in a multipath environment andbeing received by two antennas in an antenna diversity system, andmeasuring a signal quality parameter according to the presentdisclosure.

FIG. 7 shows a data format for an exemplary signal packet.

FIG. 8 shows a block diagram of an antenna diversity system thatcomputes a signal quality parameter to be used in selecting an antennafor reception, according to the present disclosure.

FIG. 9 shows a flow chart that illustrates first algorithm for selectingan antenna for reception in an antenna diversity system having twoantennas, according to the present disclosure.

FIG. 10 shows a flow chart that illustrates a second algorithm forselecting an antenna for reception in an antenna diversity system havingtwo antennas, according to the present disclosure.

FIG. 11 shows a flow chart that illustrates a process using the firstalgorithm for selecting an antenna for reception in an antenna diversitysystem having more than two antennas, according to the presentdisclosure.

FIG. 12 shows a flow chart that illustrates a process using the secondalgorithm for selecting an antenna for reception in an antenna diversitysystem having more than two antennas, according to the presentdisclosure.

FIGS. 13 and 14 show two graphs of signal-to-noise ratios for eachantenna in an exemplary seven-antenna diversity array.

FIG. 15 shows a graph of signal quality values for each antenna forwhich the signal-to-noise ratio was within a threshold value of themaximum in FIG. 14.

DESCRIPTION

The present inventors have recognized that in a multipath signalenvironment, the system performance may depend more upon characteristicsrelated to the multipath nature of the environment than upon the SNR.Hence, to optimize system performance, multipath factors are taken intoaccount. Accordingly, the present disclosure provides a method ofselecting a receive antenna in an antenna diversity system for wirelesscommunications which is based upon a signal quality measurement thatdepends primarily on multipath-related factors.

Referring to FIG. 4, a sequence of three digital symbols is shown as itwould appear after de-spreading in a signal environment having no noiseand no interference, including no interference induced by multipatheffects. Referring to FIG. 5, an exemplary sampled digital symbol thathas been affected by multipath is shown. It may be observed that thedigital symbol appears to be spread out and less clearly distinguishableas a +1. This effect can cause intersymbol interference with subsequent(or prior) transmitted symbols.

The inventive technique involves the use of a direct sequence spreadspectrum (DSSS) type of modulation. DSSS modulations use a “spreading”code, such as an 11-chip Barker code, to effectively spread the spectrumof the data. The 11-chip Barker code has a very good autocorrelationproperty, and is generally a preferred direct sequence spread spectrummodulation in the header. This property enables the data to be recoveredat very low error rates in multipath environments. The 11-chip Barkercode is as follows:

-   -   [+ − + + − + + + − − − −]        The technique is applicable for oversampled or non-oversampled        systems. In one oversampling example, each symbol is sampled 22        times. The magnitudes of the i^(th) sample may be designated as        y_(i). The sample having the largest magnitude, either positive        or negative, is designated the “peak” value y_(peak); for        example, referring again to FIG. 5, the peak 505 is represented        by the sample that most closely approximates the maximum value        for that symbol.

A signal quality (“SQ”) measurement may be computed using Equation 1below:

${SQ}_{j} = \left( \frac{y_{peak}}{{{\sum\limits_{i = 0}^{N}y_{i}} - {2y_{peak}}}} \right)$where N=the number of samples and SQ_(j)=the signal quality measurementfor the j^(th) antenna. Equation 1 defines a measurement of apeak-to-average ratio within a given data symbol, rather than asignal-to-noise ratio. Hence, the peak-to-average ratio, as formulatedabove, is much more effective at dealing with the intersymbolinterference that is a likely result of a multipath signal environment.The operation of Equation 1 above is carried out for each incomingspreading codeword in a prescribed period of time during the preamble ofeach data packet for the antennas in the antenna diversity array. Then,an antenna is selected based on either SQ, SNR, or both SQ and SNR.

Referring to FIG. 6, two illustrations of a signal propagating in amultipath environment and being received by two antennas in an antennadiversity array is shown. The top illustration 615 corresponds to anamplification factor as applied by the VGA, and the bottom illustration620 corresponds to a signal strength measurement. The region 605corresponds to the signal as received by the first antenna A₁, and thesecond region 610 corresponds to the signal as received by the secondantenna A₂. A signal quality measurement is taken for each of the twoantennas, according to Equation 1 above. In FIG. 6, the antenna A₂appears to have a higher SNR, but A₂ may or may not be the antenna to beselected.

Referring to FIG. 7, a data packet format of a typical signal is shown.The first portion 705 of the packet is a preamble, which includes aknown data pattern. The fact that the data pattern is known in advanceallows the system to perform several functions, includingsynchronization, carrier acquisition, timing acquisition, and antennaselection. A typical duration for a long preamble may be approximately128 μs; for a short preamble, it may be approximately 56 μs. The secondportion 710 of the packet is a sync frame delimiter, which simplydefines the end of the preamble. The third portion 715 is a header,which includes information about the data in the packet itself, such asthe modulation being employed (e.g., Barker code modulation versus someother DSSS modulation) and the data rate. Typical data rates for Barkercode modulation are 1 Mb/s and 2 Mb/s. The header may also include acyclic redundancy check (CRC) as a method of error detection. Finally,the last portion 720 of the packet is the actual data.

Referring to FIG. 8, a block diagram is shown for a wirelesscommunication system 800 that implements the signal quality measurementfor selecting an antenna from an antenna diversity array in a multipathenvironment. The system 800 has differences as compared to theconventional system 300 shown in FIG. 3. The system 800 takes the outputsignal of the system 300 and performs two additional functions: First,the signal passes through a spread-spectrum correlator 805 thatcorrelates the preamble of a data packet with the spreading code. Thecorrelator 805 may include a demodulator. Then, the signal qualitymeasurement is taken for each antenna in the diversity array in theprocessor 810. The processor 810 acts as a hardware implementation ofthe formula shown in Equation 1 above. The two outputs correspond to thesignal quality measurements for the two antennas. If the diversity arrayhas more than two antennas, then the number of outputs is equal to thenumber of antennas in the array. Two specific applications for theantenna diversity selection method disclosed herein are the IEEE 802.11WLAN communication system and the IEEE 802.11(b) WLAN communicationsystem, the specifications for each of which are incorporated herein byreference.

In a preferred embodiment, the antenna selection method uses both SNRmeasurements and signal quality measurements to choose an antenna on apacket-by-packet basis. The output of the AGC 345 is sent through thespreading-code correlator 805 and the signal quality measurementprocessor 810, but it is also sent through an SNR measurement processor815. When the measured SNR at a first antenna is significantly greaterthan the measured SNR at a second antenna, the antenna selection may bemade at decision unit 820 on the basis of this difference, and the firstantenna is selected. When the difference in SNR between the first andsecond antennas is less than a prescribed threshold, the signal qualitymeasurements may be used by decision unit 820 to determine which antennawill provide superior performance for that data packet, and the antennawith the highest signal quality value is selected.

Referring to FIG. 9, a flow chart illustrates a first algorithm used inthe antenna diversity selection method of the present disclosure. InFIG. 9, it is assumed that the antenna diversity system includes exactlytwo antennas. First, at step 905, the antennas are scanning to determinewhether an incoming signal is present. At step 910, a signal is detectedby one of the antennas. At step 915, the SNR and signal quality aremeasured for the antenna that has detected the signal. At step 920, theSNR and signal quality are measured for the other antenna (i.e., theantenna that did not originally detect the signal at step 910). At step925, the SNR values measured for the two antennas are compared. If thedifference between the two SNR values exceeds a predetermined threshold,then the process continues to step 930, and the antenna corresponding tothe greater SNR is selected. On the other hand, if at step 925 thedifference between the two SNR values does not exceed the predeterminedthreshold, then the process continues to step 935, and the antennacorresponding to the greater signal quality measurement is selected. Ineither case, the next step is step 940, at which a determination is madeas to whether a full data packet has been received. If not, the processsimply holds until the full data packet has been received. Once the fulldata packet has been received, the process returns to the beginning atstep 905.

The threshold value may be programmable or selectable by a systemoperator. Typically, it is expected that the usual scenario will requirethe use of the signal quality measurements as the determining factor inselecting an antenna, because the SNR usually does not vary greatly fromone antenna to the other. However, if the threshold value chosen is low,then the SNR may still be used often as the determining factor.Regardless of whether SNR or signal quality is used to select anantenna, the process is restarted for each new incoming data packet.

In an alternative embodiment, a second algorithm may be used. Referringagain to FIG. 9, the portion 950 of the figure included within thedotted line may be replaced by the steps shown in FIG. 10. As in thefirst algorithm, at step 925, an SNR difference as compared to a firstthreshold is computed, and if the SNR difference is less than the firstthreshold, the antenna having the greater SNR is selected at step 930.However, before proceeding to step 935, an additional step 932 isperformed by comparing a difference in the signal quality values for thetwo antennas to a second threshold. If the difference between the twosignal quality values is less than the second threshold, then theantenna having the higher SNR is selected at step 930; but if thedifference between the two signal quality values is greater than thesecond threshold, then the antenna having the higher signal qualityvalue is selected. In this alternative, SNR may be used to select anantenna if either the SNR difference exceeds the first threshold or ifthe signal quality difference is less than the second threshold. In thismanner, it is recognized that in some circumstances, the signal qualitymeasurement may not draw a definitive distinction between the tworeceive antennas, in which case it may be preferable to use theconventional SNR determination. Of course, if it is true that both theSNR difference is less than the first threshold and the signal qualitydifference is greater than the second threshold, the signal qualitymeasurement will be used to make an antenna selection.

The two algorithms illustrated in FIGS. 9 and 10 may be extended for usein antenna diversity systems having more than two antennas, asillustrated in FIGS. 11 and 12, respectively. Referring to FIG. 11,first, at step 1105, the antennas are scanning to determine whether anincoming signal is present. At step 1110, a signal is detected by one ofthe antennas. At step 1115, the SNR is measured for each antenna, and adetermination is made as to which antenna corresponds to the maximumSNR. This antenna is referred to as Antenna n, or the n^(th) antenna. Atstep 1120, a group of M antennas for which the SNR is within a firstthreshold of the SNR of Antenna n is determined. At step 1125, if M=0,then Antenna n is selected at step 1130. In this manner, the antennahaving the maximum SNR is always selected when its SNR exceeds the SNRsof other antennas by a significant margin.

If M is at least one, then at step 1135, the signal quality values aremeasured for the M antennas, and a determination is made as to which ofthese antennas corresponds to the maximum signal quality value. Thisantenna is referred to as Antenna q, or the q^(th) antenna. The signalquality value for Antenna n is also measured at step 1140. At step 1145,the signal quality values of Antenna n and Antenna q are compared. Ifthe signal quality value of Antenna n is greater than that of Antenna q,then Antenna n is selected at step 1140. On the other hand, if thesignal quality value of Antenna q is greater than that of Antenna n,then Antenna q is selected at step 1150. At step 1155, the process holdsuntil the entire data packet has been received, and then the processreturns to the beginning at step 1105.

An extension of the second algorithm to the case of an antenna diversitysystem having more than two antennas is illustrated in FIG. 12.Referring again to FIG. 11, the portion 1170 of the figure includedwithin the dotted line may be replaced by the steps shown in FIG. 12. Asin the first algorithm, at step 1125, if it is determined that thedifference between the SNR of Antenna n and the SNR of each of the otherantennas is greater than the first threshold (i.e., M=0), then Antenna nis selected at step 1130. If M is at least one, then at step 1136, thesignal quality values are measured for the M antennas and Antenna n, anda determination is made as to which of these antennas corresponds to themaximum signal quality value. This antenna is referred to as Antenna r,or the r^(th) antenna. The next step, step 1138, determines which of thegroup of M antennas and Antenna n corresponds to the second-highestsignal quality value. This antenna is referred to as Antenna p, or thep^(th) antenna. At step 1146, the difference between the signal qualityvalues of Antenna r and Antenna p is computed and then compared to asecond threshold value. If this difference is less than the secondthreshold value, then Antenna n is selected at step 1130. On the otherhand, if the signal quality value of Antenna r exceeds the signalquality values of Antenna p by more than the second threshold, thenAntenna r is selected at step 1152.

Referring to FIGS. 13, 14, and 15, the two algorithms described aboveare illustrated for an exemplary antenna diversity array having sevenantennas. In FIG. 13, Antenna 4 corresponds to the maximum SNR, and thefirst threshold value is illustrated by the dotted lines. In FIG. 13,none of the other six antennas in the array corresponds to an SNR thatfalls within the threshold region. Thus, in this case, Antenna 4 will beselected by both algorithms, and the signal quality values will not beused for the antenna selection.

Referring to FIG. 14, Antenna 1 corresponds to the maximum SNR, andAntenna 3 and Antenna 5 also correspond to SNRs that fall within thethreshold region. Therefore, in this case, the signal quality valueswill be measured for Antennas 1, 3, and 5, as shown in FIG. 15. Antennas2, 4, 6, and 7 are effectively eliminated from the selection processbecause their SNR values do not fall within the threshold region.

Referring to FIG. 15, Antenna 3 corresponds to the maximum signalquality value, and Antenna 5 corresponds to the second-highest signalquality value. As described above, when the first algorithm is beingemployed (i.e., see FIG. 11), the antenna having the maximum signalquality value of those falling within the threshold region for SNR ischosen. Therefore, in the case illustrated in FIG. 15, Antenna 3 will beselected when the first algorithm is being employed.

When the second algorithm is being employed, as described above withreference to FIG. 12, a second threshold region is applied, and if thesecond-highest signal quality value is within the second thresholdregion, then the antenna having the maximum SNR is selected. In the caseillustrated in FIG. 15, the signal quality value for Antenna 5 does fallwithin the second threshold region. Therefore, when the second algorithmis employed, the antenna having the maximum SNR, i.e., Antenna 1, willbe selected. In this manner, it is recognized that in thesecircumstances, the signal quality measurement may not draw a definitivedistinction between the receive antennas (i.e., Antennas 1, 3 and 5), inwhich case it may be preferable to use the conventional SNRdetermination.

The equation used for calculating signal quality, Equation 1 above, maybe specified more exactly to correspond to the non-oversampled case andthe oversampled case, respectively. Supposing that an N-chip spreadingcode is used and a non-oversampled system is being employed, there willbe one sample for each chip, thus N samples for each symbol, which maybe represented as (y₀, y₁, y_(N−1)). Let p denote the index of thesample corresponding to the value having the peak magnitude; hence,|y_(p)|=max_(i)|y_(i)|. Then, for any arbitrary values of L and M, thesignal quality measurement equation may be expressed as follows(Equation 2):

${SQ} = \left( \frac{y_{p}}{\sum\limits_{i = 1}^{L}\left( {{y_{p - M - i}} + {y_{p + M + i}}} \right)} \right)$where the index arithmetic is performed modulo N, i.e.,y_(p−M−i)=y_((p−M−i)mod N) and y_(p+M+i)=y_((p+m+i)mod N).

As an example of an oversampled system using an N-chip spreading code,suppose that 2N samples are taken for each symbol; therefore, the symbolmay be represented as (y₀, y₁, . . . , y_(2N−1)). Again, let p denotethe index of the sample corresponding to the value having the peakmagnitude; hence, |y_(p)|=max_(i)|y_(i)|. Then, for any arbitrary valuesof L and M, the signal quality measurement equation may be expressed asfollows (Equation 3):

${SQ} = \left( \frac{y_{p}}{\sum\limits_{i = 1}^{L}\left( {{y_{p - M - i}} + {y_{p + M + i}}} \right)} \right)$where the index arithmetic is performed modulo 2N, i.e.,y_(p−M−i)=y_((p−M−i)mod 2N) and y_(p+M+i)=y_((p+M+i)mod 2N). It is notedthat the case of the oversampled system is preferred.

The signal quality measurement formula may also be modified by using amoving average of signal quality calculations, which may be effected bysimply adding several signal quality measurements together. Thetechnique of using the signal quality measurement may also be applied toantenna diversity systems using more than two antennas, for example,three-antenna systems, four-antenna systems, or larger antenna arrays.Various implementations of the technique of using the signal qualitymeasurement to select an antenna may be used. Such implementationsinclude systems having appropriate circuitry, methods of usingcommunication system circuitry, and storage media having software thatincludes instructions for causing a computer to execute the signalquality measurement technique.

As an example, it is to be understood that although an implementationuses the IEEE 802.11(b) WLAN with the 11-chip Barker code, thetechniques disclosed herein is applicable to other wirelesscommunications systems that use other types of spreading codes anddirect sequence spread-spectrum modulations. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

What is claimed is:
 1. A system comprising: a receiver configured toreceive (i) a first signal via a first antenna, and (ii) one of thefirst signal and a second signal via a second antenna; at least oneprocessor configured to determine (i) based on the first signal asreceived at the first antenna, a first signal-to-noise ratio and a firstsignal quality value, and (ii) based on the one of the first signal andthe second signal as received at the second antenna, a secondsignal-to-noise ratio and a second signal quality value, wherein thefirst signal-to-noise ratio is different than the first signal qualityvalue, and wherein the second signal-to-noise ratio is different thanthe second signal quality value; and a decision device configured toselect one of the first antenna and the second antenna based on adifference between the first signal-to-noise ratio and the secondsignal-to-noise ratio if the first signal-to-noise ratio is greater thanthe second signal-to-noise ratio, or the first signal quality value andthe second signal quality value if the difference between the firstsignal-to-noise ratio and the second signal-to-noise ratio is less thana predetermined threshold.
 2. The system of claim 1, wherein the atleast one processor comprises: a first processor configured to determine(i) based on the first signal as received at the first antenna, thefirst signal-to-noise ratio, and (ii) based on the one of the firstsignal and the second signal as received at the second antenna, thesecond signal-to-noise ratio; and a second processor configured todetermine (i) based on the first signal as received at the firstantenna, the first signal quality value, and (ii) based on the one ofthe first signal and the second signal as received at the secondantenna, the second signal quality value.
 3. The system of claim 1,wherein: the first signal quality value is a first peak-to-averageratio; and the second signal quality value is a second peak-to-averageratio.
 4. The system of claim 3, wherein the decision device isconfigured to select (i) the first antenna if the first peak-to-averageratio is greater than the second peak-to-average ratio, and (ii) thesecond antenna if the second peak-to-average ratio is greater than thefirst peak-to-average ratio.
 5. The system of claim 1, wherein thedecision device is configured to select (i) the first antenna if thefirst signal quality value is greater than the second signal qualityvalue, and (ii) the second antenna if the second signal quality value isgreater than the first signal quality value.
 6. The system of claim 1,wherein the decision device is configured to: determine which one thefirst signal-to-noise ratio and the second signal-to-noise ratio islarger; and if the difference is greater than the predeterminedthreshold, select the first antenna if the first signal-to-noise ratiois greater than the second signal-to-noise ratio, and select the secondantenna if the second signal-to-noise ratio is greater than the firstsignal-to-noise ratio.
 7. The system of claim 1, wherein: the receiveris configured to receive the second signal via the second antenna; theat least one processor is configured to determine, based on the secondsignal as received at the second antenna, the second signal-to-noiseratio and the second signal quality value, and determine which one thefirst signal-to-noise ratio and the second signal-to-noise ratio islarger; and the decision device is configured to, if the difference isgreater than the predetermined threshold, select one of the firstantenna and the second antenna based on the first signal-to-noise ratioand the second signal-to-noise ratio.
 8. The system of claim 7, whereinthe decision device is configured to, if the difference is greater thanthe predetermined threshold: select the first antenna if the firstsignal-to-noise ratio is greater than the second signal-to-noise ratio;and select the second antenna if the second signal-to-noise ratio isgreater than the first signal-to-noise ratio.
 9. The system of claim 1,wherein: the at least one processor is configured receive a plurality ofsignals, each of the plurality of signals is received by a respectiveone of a plurality of antennas, wherein the plurality of signals includethe first signal and the second signal, and wherein the plurality ofantennas include the first antenna and the second antenna, and determinewhich one of the plurality of signals has a largest signal-to-noiseratio; and the decision device is configured to select the one of theplurality of antennas receiving the one of the plurality of signals withthe largest signal-to-noise ratio.
 10. The system of claim 1, wherein:the at least one processor is configured to receive a plurality ofsignals, each of the plurality of signals is received by a respectiveone of a plurality of antennas, wherein the plurality of signals includethe first signal and the second signal, and wherein the plurality ofantennas include the first antenna and the second antenna, and determinewhich one of the plurality of signals has a largest signal-to-noiseratio, and differences between (i) the largest signal-to-noise ratio ofthe one of the plurality of signals, and (ii) signal-to-noise ratios ofother ones of the plurality of signals; and the decision device isconfigured to, if each of the differences is greater than thepredetermined threshold, select the one of the plurality of antennaswhich received the one of the plurality of signals with the largestsignal-to-noise ratio.
 11. The system of claim 10, wherein: the at leastone processor is configured to determine a plurality of signal qualityvalues for the plurality of signals, wherein the plurality of signalquality values include the first signal quality value and the secondsignal quality value; and the decision device is configured to, if oneof the differences is less than the predetermined threshold, select theone of the plurality of antennas based on the plurality of signalquality values.
 12. The system of claim 1, wherein: the at least oneprocessor is configured to receive a plurality of signals, each of theplurality of signals is received by a respective one of a plurality ofantennas, wherein the plurality of signals include the first signal andthe second signal, and wherein the plurality of antennas include thefirst antenna and the second antenna, and determine which one of theplurality of signals has a largest signal quality value, and differencesbetween (i) a largest signal quality value of the one of the pluralityof signals, and (ii) signal quality values of other ones of theplurality of signals; and the decision device is configured to, if eachof the differences is greater than a second predetermined threshold,select the one of the plurality of antennas which received the one ofthe plurality of signals with the largest signal quality value.
 13. Thesystem of claim 12, wherein: the at least one processor is configured todetermine which one of the plurality of signals has a largestsignal-to-noise ratio; and the decision device is configured to, if oneof the differences is less than the second predetermined threshold,select the one of the plurality of antennas which received the one ofthe plurality of signals with the largest signal-to-noise ratio.
 14. Amethod comprising: receiving (i) a first signal via a first antenna, and(ii) one of the first signal and a second signal via a second antenna;determining (i) based on the first signal as received at the firstantenna, a first signal-to-noise ratio and a first signal quality value,and (ii) based on the one of the first signal and the second signal asreceived at the second antenna, a second signal-to-noise ratio and asecond signal quality value, wherein the first signal-to-noise ratio isdifferent than the first signal quality value, and wherein the secondsignal-to-noise ratio is different than the second signal quality value;and selecting one of the first antenna and the second antenna based on adifference between the first signal-to-noise ratio and the secondsignal-to-noise ratio if the first signal-to-noise ratio is greater thanthe second signal-to-noise ratio, or the first signal quality value andthe second signal quality value if the difference between the firstsignal-to-noise ratio and the second signal-to-noise ratio is less thana predetermined threshold.
 15. The method of claim 14, wherein: thefirst signal quality value is a first peak-to-average ratio; and thesecond signal quality value is a second peak-to-average ratio.
 16. Themethod of claim 14, further comprising: determining which one the firstsignal-to-noise ratio and the second signal-to-noise ratio is larger;and if the difference is greater than the predetermined threshold,selecting the first antenna if the first signal-to-noise ratio isgreater than the second signal-to-noise ratio, and selecting the secondantenna if the second signal-to-noise ratio is greater than the firstsignal-to-noise ratio.
 17. The method of claim 14, further comprising:receiving the second signal via the second antenna; determining, basedon the second signal as received at the second antenna, the secondsignal-to-noise ratio and the second signal quality value; anddetermining which one the first signal-to-noise ratio and the secondsignal-to-noise ratio is larger; and if the difference is greater thanthe predetermined threshold, selecting one of the first antenna and thesecond antenna based on the first signal-to-noise ratio and the secondsignal-to-noise ratio.
 18. The method of claim 14, further comprising:receiving a plurality of signals, wherein each of the plurality ofsignals is received by a respective one of a plurality of antennas,wherein the plurality of signals include the first signal and the secondsignal, and wherein the plurality of antennas include the first antennaand the second antenna; determining which one of the plurality ofsignals has a largest signal-to-noise ratio; and selecting the one ofthe plurality of antennas receiving the one of the plurality of signalswith the largest signal-to-noise ratio.
 19. The method of claim 14,further comprising: receiving a plurality of signals, wherein each ofthe plurality of signals is received by a respective one of a pluralityof antennas, wherein the plurality of signals include the first signaland the second signal, and wherein the plurality of antennas include thefirst antenna and the second antenna, and determining which one of theplurality of signals has a largest signal-to-noise ratio, anddifferences between (i) the largest signal-to-noise ratio of the one ofthe plurality of signals, and (ii) signal-to-noise ratios of other onesof the plurality of signals; and if each of the differences is greaterthan the predetermined threshold, selecting the one of the plurality ofantennas which received the one of the plurality of signals with thelargest signal-to-noise ratio.
 20. The method of claim 14, furthercomprising: receiving a plurality of signals, wherein each of theplurality of signals is received by a respective one of a plurality ofantennas, wherein the plurality of signals include the first signal andthe second signal, and wherein the plurality of antennas include thefirst antenna and the second antenna, and determining which one of theplurality of signals has a largest signal quality value, differencesbetween (i) a largest signal quality value of the one of the pluralityof signals, and (ii) signal quality values of other ones of theplurality of signals; and if each of the differences is greater than asecond predetermined threshold, selecting the one of the plurality ofantennas which received the one of the plurality of signals with thelargest signal quality value.